The present invention is concerned with the growth of mercury cadmium telluride. In particular, the present invention is directed to the reduction of compositional gradients in mercury cadmium telluride. For the purposes of this specification, the common chemical equations for mercury cadmium telluride, (Hg,Cd)Te or Hg.sub.1.sub.-x Cd.sub.x Te, will be used.
(Hg,Cd)Te is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a wide gap semiconductor (E.sub.g =1.6 eV), with mercury telluride, which is a semi-metal having a "negative energy gap" of about -0.3 eV. The energy gap of the alloy varies linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting x, it is possible to obtain (Hg,Cd)Te detector material having a peak response over a wide range of infrared wavelengths.
(Hg,Cd)Te is of particular importance as a detector material for the important 8 to 14 micron atmospheric transmission "window". Extrinsic photoconductor detectors, notably mercury doped germanium, have been available with high performance in the 8 to 14 micron wavelength interval. These extrinsic photoconductors, however, require very low operating temperatures (below 30.degree.K). (Hg,Cd)Te intrinsic photodetectors having a spectral cutoff of 14 microns, on the other hand, are capable of high performance at 77.degree.K.
The possible application of (Hg,CD)Te as an intrinsic photodetector material for infrared wavelengths was first suggested by W. G. Lawson et al., J. Phys. Chem. Solids, 9, 325 (1959). Since that time extensive investigation of (Hg,Cd)Te has been performed. High performance (Hg,Cd)Te detectors have been achieved for wavelengths from about 1 to 30 microns.
Despite the potential advantages of (Hg,Cd)Te as an infrared detector material, (Hg,Cd)Te photodetectors have only recently found wide use in infrared detector systems. The main drawback of (Hg,Cd)Te has been the difficulty in preparing high quality, uniform material in a consistent manner. The preparation of (Hg,Cd)Te crystals having n-type conductivity, which is the desired conductivity type for photoconductive detectors, has been found to be particularly difficult.
Several properties of the Hg-Cd-Te alloy system cause the difficulties which have been encountered in preparing (Hg,Cd)Te. First, the phase diagram for the alloy shows a marked difference between the liquidus and solidus curves, thus resulting in segregation of CdTe with respect to HgTe during crystal growth. Conventional crystal growth methods, which involve slow cooling along the length of an ingot, produce an extremely inhomogenous body of (Hg,Cd)Te. Second, the high vapor pressure of Hg over the melt makes it difficult to maintain melt stoichiometry. Third, the segregation of excess Te can give rise to pronounced constitutional supercooling.
The crystal preparation technique which has been most successful in producing high quality (Hg,Cd)Te is the technique described by P. W. Kruse et al. in U.S. Pat. No. 3,723,190. This technique involves the bulk growth of homogenous (Hg,Cd)Te alloy crystals by a three part method. First, a liquid solution of the desired alloy composition is quenched to form a solid body of (Hg,Cd)Te. Second, the body is annealed at a temperature near but below the solidus temperature to remove dendrites. Third, the (Hg,Cd)Te is annealed at low temperature in the presence of excess Hg to adjust stoichiometry. This final low temperature anneal takes about 30 days.
The second step of the three step process is required because the rapid solidification of the first step produces a largely single crystal ingot containing a dendritic structure, with alternating regions of high and low mole ratios. As the cooling rate is increased, the dendritic structure becomes finer. The subsequent high temperature anneal at a temperature below the solidus temperature is required to remove the dendrites. While it is possible to remove the dentritic microscopic compositional gradients, which are generally less than one millimeter, the high temperature annealing step takes weeks and leaves macroscopic x gradients on the order of several millimeters unaffected. The removal of macroscopic x gradients by this method takes months or even years and is, therefore, impractical.